1. Field of the Invention
This invention relates to a method and apparatus for protection of signal copy intended to prevent unauthorized copy of music software such as record, compact disk (CD) and music tape by recording.
2. Description of the Prior Art
Recently, with the advent of DAT (Digital Audio Tape-recorders), it has come to be possible to copy the music software with a high quality. To the contrary, the music software manufactures wish to introduce a system to prevent copying of music software by a recorder.
Formerly, as one of such systems, a copy code system disclosed by CBS was known. This system is described below.
In the manufacturing process of music software, that is, in the step of recording audio signals onto a recording medium such as a record, a CD and music tape, the audio signals recorded in the recording medium have their signal level attenuated level in a specific band by a band elimination filter. The center frequency of this band elimination filter is 3840 Hz, and the band is 250 Hz wide.
In the process of recording by reproducing this recording medium, the reproduction signal is respectively fed into two bandpass filters. The first bandpass filter has a center frequency at 3840 Hz, and the second bandpass filter, has a center frequency near 3840 Hz. Comparing the output amplitudes of these two bandpass filters, when the first bandpass filter is lower than the second bandpass filter, it is judged that the signal level of a specific band has been attenuated at the time of fabrication of the software, and the recording action is stopped. That is, by attenuating the signal in a specific band at the time of fabrication of the music software, copying by recording is prevented.
In such a constitution, however, an inaudible sound is present for attenuating a specific band of audio signals, or the sound quality changes due to large variations of the phase characteristic in the vicinity of the attenuation band of the band elimination filter. Or, in a music source originally small in the signal components of 3840 Hz, there was a possibility of malfunction. Thus, the conventional method has various problems.